Have you ever heard people say they feel like the weight of the world is on their shoulders? When people say that, they're expressing weariness from feeling overburdened by stressful events in their lives. After all, the actual weight of the world would crush even the strongest superheroes, wouldn't it?

You bet it would! Think about the things that you can lift. Your pencil? No problem! Your backpack? It might be heavy on a day with a lot of homework, but it's no big deal. But what about the school bus you ride? Nope! You're not lifting that even an inch off the ground.

Now think about how big Earth is. Have you ever seen a huge rocky mountain? Can you imagine how many millions of pounds it must weigh? Now imagine all the mountains in the world…and realize that those are only a tiny fraction of all the “stuff" that makes up Earth.

It's really mind-boggling, isn't it? Is there even a way to weigh Earth? It's not like there's a big scale floating out there in space between Earth and Mars that our planet can step onto to see how much it weighs. So how could we ever learn how heavy our world is?

Leave it to the mathematicians and scientists to use what they know about Earth and the laws of physics to figure out how heavy Earth is. Of course, the first thing they'll tell you is that they estimate Earth's mass, not its weight.

Weight is actually a measure that is dependent upon a gravitational field to determine. For example, you weigh six times more on Earth than you would on the Moon, because Earth's gravitational force is six times that of the Moon.

To measure Earth's weight, one would need to calculate it based upon Earth being in the gravitational field of another object, such as the Sun or another planet. Instead, scientists and mathematicians estimate Earth's mass, which is a measure of the amount of matter that makes up Earth and is a constant that doesn't depend upon a gravitational field.

The force of gravity is still involved in estimating Earth's mass, however! The laws of physics state that any two objects will have a gravitational attraction between them. For two spherical objects, scientists and mathematicians have developed a formula to calculate this force.

Using this formula, you can figure out the mass of one of the spherical objects if you know the radius of that object and the mass of the other object. Scientists know that Earth's radius is 6,999,125 yards. Using a simple sphere with a known mass, scientists and mathematicians can then calculate Earth's estimated mass.

After completing their calculations and double-checking their numbers with a calculator or a computer, scientists and mathematicians have concluded that Earth's mass is approximately 13,170,000,000,000,000,000,000,000 pounds! In scientific notation, that's 1.3 x 10^{25} pounds.

Can you believe it? Is Earth fat? Nope! Earth is simply the largest of the terrestrial or rocky planets in our solar system. The fact that Earth's surface and mantle are about one-third iron and Earth's core is over 90% iron makes Earth a very heavy planet indeed!